Console warning should point readers to MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of

Log console warnings for JavaScript 1.6's deprecated for-each-in loops. As seen in Beta 34 telemetry, fewer than 250 for-each-in loops were reported web content (though for-each-in loops are popular in Firefox chrome code): http://telemetry.mozilla.org/#filter=beta%2F34%2FJS_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT&aggregates=multiselect-all!Submissions&evoOver=Time&locked=true&sanitize=true&renderhistogram=Table 1. This patch logs the same warning for JavaScript 1.6's for-each-in loops and JavaScript 1.7's for-each-in array comprehensions. The preferred fix for these legacy array comprehensions is to use ES7 array comprehensions or something like Array.map(). Is it counterproductive to warn developers about for-each-in in legacy array comprehensions instead of telling them to not use legacy array comprehensions? 2. To reduce console noise, this patch only warns for JS code that is *not* JSVERSION_LATEST (i.e. JS 1.8.5, the default chrome JS version). I have only found a few instances of Firefox code using for-each-in with JS version 1.7 or 1.8. Later we can restrict for-each-in to just chrome JS.

https://hg.mozilla.org/integration/mozilla-inbound/rev/7d0b5801159a https://hg.mozilla.org/integration/mozilla-inbound/rev/7a3e8ce41a29

https://hg.mozilla.org/mozilla-central/rev/7d0b5801159a https://hg.mozilla.org/mozilla-central/rev/7a3e8ce41a29

Added to the compat doc: https://developer.mozilla.org/en-US/Firefox/Releases/37/Site_Compatibility